Nowadays, the majority of beverage containers are lightweight plastic bottles of a material such as PET (polyethylene terephthalate), manufactured in a two-step process. In a first step, so-called hollow “preforms” are created from the raw material, e.g. by a slow, high-temperature extrusion process. In a second step the ‘walls’ of these hollow preforms are heated again—this time to a temperature below their re-crystallization point, i.e. below about 130° C.—and then formed into the desired bottle-shapes via blow-moulding.
In most state of the art preform heating ovens, this heating is carried out using halogen lamps, for which a significant portion of the broad emission spectrum lies in the infrared region. During the heating process, heat energy is ‘deposited’ in the preform material. The infrared heating technique, while technically well-developed, has the drawback of limited energy efficiency. One reason for the poor energy efficiency is that the radiation emitted by the halogen lamps cannot effectively be directed or focused. The main reason, however, is the poor matching of the emission spectrum of the halogen lamps to the absorption spectrum of the preform material, which results in absorption of energy in inappropriate regions of the preform, for example in its outer surface. This results in higher temperatures in some regions of the preform wall, while other regions are insufficiently heated. For example the outside surface of the preform may become very hot while the inside surface is insufficiently heated. Furthermore, the distribution of deposited energy inside the preform wall does not necessarily result in the same distribution of temperatures on account of the thermal conductivity of the material. Such temperature gradients or hot spots make it difficult to ensure an even quality of the subsequent blow-moulding stage, and may even result in damage to the outer surface of the preform. Therefore, some prior art processes even resort to an additional cooling of the preforms during the heating-process (for example by forced-air cooling); a measure which is obviously costly and inefficient from the point of view of energy consumption, and therefore undesirable. Alternatively, in prior art ovens, an unwanted temperature gradient may be dealt with by “equilibration phases”, i.e. time-delays during which thermal conduction inside the preform material should lead to an equalization of the temperatures in the preform wall. These equilibration phases typically take several seconds (up to ten). However, including these phases into the heating process means that the total process time is lengthened, thereby increasing the overall cost of the preform heating process.